This invention relates to polymeric compositions comprising polyisoprene, polybutadiene and a diblock copolymer of polyisoprene and polybutadiene.
Presently, blends of two or more polymers are utilized in a wide range of applications in order to obtain the characteristics of each of the components for the overall composition. In a specific application, elastomeric blends of polyisoprene and polybutadiene are utilized to make tires wherein polybutadiene provides skid and abrasion resistance while polyisoprene provides tensile and tear strength as well as crack resistance to the overall composition. Polybutadiene is not usually used by itself because of its poor processability and it usually amounts up to about 40 weight percent in blends with polyisoprene.
In polymer blends, imcompatibility of chemically dissimilar polymers is the rule and compatibility is the exception. In other words, blends of two or more polymers are hetrogeneous except in some special cases. For purposes of this disclosure, a blend is defined as hetrogeneous if mixing at the molecular level is not achieved. In order to avoid the confusion often found in the literature in describing two-phase polymer blends, the expression "degree of hetrogeneity" is defined as follows: The degree of hetrogeneity of a two-phase polymer blend decreases as the size of the characteristic domains in the blend decreases.
Prior to the present invention, diblock copolymers have been utilized to reduce the degree of hetrogeneity of blends of highly incompatible homopolymers. The mechanism of the formation of domains of each of the homopolymers is explained by proposing that the block copolymers have an emulsifying effect on both of the homopolymers. In these polymeric compositions, separate domains of each the homopolymers employed in the blends are clearly identifiable. This is undesirable since there will be corresponding portions of the blend which will be deficient in a desirable physical characteristic as a result of the domains under consideration being deficient in one of the homopolymers. Therefore, it would be desirable to provide polymer compositions comprising a blend of one or two homopolymers with a copolymer which is free of such domains and is homogeneous even at the molecular level.